1. Field of the Invention
The present invention relates to a photoelectric switch, and more specifically relates to improvement in photoelectric switch that receives reflected light from a detection area to acquire color information and performs workpiece determination.
2. Description of Related Art
A photoelectric switch is a detector for detecting a workpiece by use of light. The photoelectric switch projects detected light, and receives light reflected by or light transmitted through a workpiece, or some other light, to perform workpiece determination. Based on the result of the workpiece determination, the photoelectric switch generates a detection signal. Types of the photoelectric switch include: a light receiving amount-type photoelectric switch that performs workpiece determination by use of a light receiving amount of reflected light or transmitted light from a detection area including a workpiece; a distance measurement-type photoelectric switch that measures a distance to the workpiece to perform workpiece determination, and a color discrimination-type photoelectric switch that performs workpiece determination by discriminating colors of the workpiece surface.
The light receiving amount-type photoelectric switch performs workpiece discrimination through use of variation in light receiving amount due to a difference in reflectance or color of the workpiece surface, a difference in distance to the workpiece, a difference in tilt (tilt angle) of the workpiece surface, or the like. This is a general-purpose photoelectric switch applicable to a large number of uses.
Meanwhile, the distance measurement-type photoelectric switch measures a characteristic in accordance with the shape of the workpiece as a distance to the workpiece, to perform the workpiece determination. This is less susceptible to variation in reflectance and color of the workpiece surface or variation in tilt (tilt angle) of the workpiece surface. Further, the color discrimination-type photoelectric switch performs the workpiece determination by use of the color of the workpiece surface. This is less susceptible to variation in reflectance of the workpiece surface, variation in distance to the workpiece, or variation in tilt (tilt angle) of the workpiece surface.
A conventional color discrimination-type photoelectric switch includes: three light emitting elements for respectively generating red, green, and blue detected light; one light receiving element for receiving reflected light to generate a light reception signal; and an optical fiber cable for leading the detected light to a light projecting position (e.g., Unexamined Japanese Patent Publication No. 2005-134363 and Unexamined Japanese Patent Publication No. H11-330940). The three light emitting elements each have a structure where a light emitting diode is mounted in a cup integrally formed with a lead frame, and its periphery is molded into a shell shape by epoxy resin or the like. The photoelectric switch further includes: a wavelength selection-type dichroic mirror for aligning light, emitted from each light emitting element, to the same axis; and a condensing lens for concentrating light, aligned to the same axis, onto an optical fiber cable. In this photoelectric switch, colors of a workpiece is discriminated based on light receiving amount levels of the three colors obtained by sequentially lighting the respective light emitting elements in a time-division manner. Further, the detected light emitted from the light emitting element is transmitted through the optical fiber cable to obtain a light projection spot with reduced light amount irregularity.
In the conventional photoelectric switch described above, due to the need for using the optical fiber cable so as to reduce the light amount irregularity (color irregularity) of the detected light, it has been difficult to reduce a size of a whole detector even if the head portion can be reduced in size. When the light amount irregularity of the detected light is large, the accuracy deteriorates in color discrimination of a workpiece smaller than a light projection spot. Further, an optical system of the photoelectric switch described above needs to include three shell-type light emitting elements, two wavelength selection-type dichroic mirrors, and one condensing lens, and the optical system thus has a large size. Hence it has been difficult to reduce the size of the whole detector.
Moreover, the optical system of the photoelectric switch described above has an advantage in heat dissipation properties due to the light emitting elements being divided into three parts. However, since it is a shell-type light emitting element, if a drive current is increased so as to obtain high light projection intensity, the problem of the heat dissipation might become apparent, and it has thus been difficult to achieve both the high light projection intensity and the size reduction. Furthermore, in the foregoing photoelectric switch, in order to make a light emission intensity ratio with respect to the three light emitting elements constant, the light emission intensity of each light emitting element is detected by a monitor PD to control the light projection intensity so as to make light emission intensity for each light emitting element constant. There has thus been a problem of a small dynamic range for detection.